I also have this problem with a LG 16x DVD-RW drive, P4 1.8 Ghz.
Tunning hdparams increased a bit the speed to 2x.
Changing the k3b process priority to 19 increased further the speed to
6x (but it is still far from 16x).
I guess the problem is related to the CPU usage: hdparams reduces CPU
usage, and renice gives more CPU to k3b, so both increase the burning
speed.
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Dapper: k3b slow dvd burn speed
https://launchpad.net/bugs/31709